Happens like clockwork. At this mileage there's a noticeable acoustic change at idle and low rpm. It goes from butter quiet to more metallic. Is this mileage at the point where a particular additive takes over/stops working?

I couldn't do it. I couldn't do 13K mile OCI's in such a nice car. Why? To save a few bucks? Not when the oil is obviously out of grade...causing your changes in engine accoustics.

Have you done any UOA's on this car?

My bet is the oil is out of grade. I bet if you drained 2 quarts out and replaced with 2 quarts of 5w40, it would sound nice and quiet again.

The noise is telling you something: the oil is out of grade (too thin). If you want to run 13k OCI's....then use a thicker oil so it will shear into grade, rather than using 5w30 and having it shear out of grade. Run a good 5w40 or even 5w50 (which are usually on the thin side of a 50 weight).

Another vote for the oil going out of grade, and has probably been in service a little too long. Either do a UOA and see what's going on, or shorten your OCI to the point just before the acoustics change.